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The Earth’sinterior structure - Lecture 1
How do we know what’s inside the Earth?• samples from inside the Earth (not many)• indirect observations of the Earth’s interior seismic, gravity, magnetic, and heat flow measurements• today (mostly): seismic studies of Earth’s interior
EARTH’S INTERIOR: SAMPLES
Plummer 1st Cdn edition, Ch. 4 pp 107-109, plus Box 4.4
(page 117)
KTB borehole -- drilled to a depth of 10 km in GermanyRussians have drilled a 12-km-deep borehole
Convecting Mantle
Kimberlites: samples of the upper mantle
Kimberlites: contain samples of the upper mantle (from >150 km deep)
Box 4.4, page 118 (Plummer 1st Canadian edition)
Mantle xenoliths: samples of the upper mantle from volcanoes
http://www.ees.nmt.edu/condie/MantleXenolith_NM.jpg http://www.calstatela.edu/faculty/acolvil/interior.html
EVIDENCE FROM SEISMIC WAVES
Plummer 1st Cdn. edition, Ch. 4 pp. 109-112
Refracted and Reflected Waves
Fig. 4.01
Seismic Reflection
Seismic Reflection
T53
Crustal Seismology: Seismic Reflection
Lithoprobe Seismic ReflectionNovember 2000
Vibroseis trucksStewart-Cassiar HwyNorthwest BC
pp 110-111, Canadian edition of text
Active sourcereflection seismology:
Petroleum industry’sprimary explorationtechnique
• energy reflects off sharp boundaries between rocks with strong contrasts in density and seismic velocity
• reflection is widely used by oil industry to find hydrocarbon traps in sedimentary basins
Seismic Reflection
Fig. 4.2
Seismic Refraction
Fig. 4.3
Effect of material change on paths of Refracted and reflected waves
layer with slow v
layer with faster v
layer with even faster v
With no change in properties, no refractions, no reflections
Slower
Faster
Curvature(refraction)
of the energy
Velocitygraduallyincreasingwith depth
seismic raypath bends
• energy is transmitted through layers and takes a curved path back to the surface without a reflection
• this happens because seismic velocity increases gradually with depth
• seismic velocities in the Earth generally increase with depth due to effects of pressure
Seismic Refraction
EARTH’S INTERNAL STRUCTURE
Plummer et al. Canadian Edition Ch. 4 pp. 112-119
Fig. 4.5
• Earth structure dominantly radial due to – pressure: rearrange atoms to form denser
minerals, for example, in the mantle
– compositional differentiation (denser elements sink to the center)
• main compositional layers are:– crust (0 to 30 km, on average)
– mantle (30 to 2900 km)
– core (2900 to 6370 km)
Earth’s Radial Structure
• some important rheologic layers are:– solid lithosphere (0 to 150 km)
– gooey asthenosphere (150 to 300 km)
– liquid outer core (2900 to 5150 km)
– solid inner core (5150 to 6370 km)
Earth’s Radial Structure
Figure 4.6
P-wave velocities in crust and upper mantleMUCH lowervelocities nearEarth’s surface
Table 4.1 in Canadian edition of text
Fig. 4.7
P- and S- wave velocities inside the Earth
• upper mantle (30-660 km)– 8 km/s : peridotite
– seismic velocity variations due to lithosphere, asthenosphere (gooey), olivine phase changes
• lower mantle (660-2900 km)– steady increase in velocity
– probably same composition as upper mantle
Wait! How do we know this??
We measure the arrival times of different seismic waves, and compare them with what we would expect from calculations for a model Earth.
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